1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to systems and methods of providing gain clamped Thulium-doped fiber amplification of optical signals.
2. Statement of the Problem
Many communication companies use fiber optic cabling as a media for transmitting data because of its high-bandwidth capacity. Fiber optic cables reliably transport optical signals over long distances. Over a distance, optical signals attenuate in the fiber due to Rayleigh scattering. The attenuation may be recovered by an optical amplifier. However, the optical amplifier adds noise to the optical signals. The noise accumulation on the optical signals can especially be a problem for ultra long haul transmissions.
Optical amplifiers may be discrete amplifiers or distributed amplifiers. Distributed amplifiers use the transmission fiber carrying the optical signals as a gain medium. Discrete amplifiers do not use the transmission fiber as a gain medium, but use another type of fiber or component as the gain medium.
One type of discrete amplifier is an Erbium-Doped Fiber Amplifier (EDFA). In an EDFA, an Erbium-doped fiber receives optical signals from a transmission fiber. A Raman fiber pump transmits light having a wavelength of 980 nm onto the Erbium-doped fiber concurrently as the optical signals travel over the Erbium-doped fiber. The properties of the Erbium-doped fiber act to absorb the pumped light and generate a gain in the optical signals using the absorbed light.
To control the gain generated by a C-band EDFA, the EDFA includes a feedback loop. A fiber Bragg grating or another type of filter separate one or more wavelengths out of the amplified optical signals. The separated wavelength or wavelengths comprise a feedback signal. The feedback loop receives the feedback signal and combines the feedback signal with the optical signals to be amplified. The feedback loop helps to clamp the gain of the EDFA at a constant level over the C-band or a portion of the C-band.
Unfortunately, traditional EDFA's have a limit on the gain bandwidth they can generate. An EDFA with a 980 nm pump amplifies the C-band. The C-band refers to optical signals having wavelengths in the range of 1530 nm to 1560 nm. The C-band may not provide enough bandwidth as the demand for capacity increases.
To increase capacity, the S-band can also be used. The S-band refers to optical signals having wavelengths in the range of 1450 nm to 1480 nm. Thulium-doped fiber amplifiers (TDFA) have been developed to amplify the S-band. In a TDFA, a Thulium-doped fiber receives optical signals from a transmission fiber. A Raman fiber pump transmits light onto the Thulium-doped fiber concurrently as the optical signals travel over the Thulium-doped fiber. The properties of the Thulium-doped fiber act to absorb the pumped light and generate a gain in the S-band using the absorbed light.
Inherent properties of the Thulium-doped fiber make it difficult to control the gain generated by the TDFA. In one prior art TDFA, three pumps (1421 nm, 1427 nm, and 1434 nm) are used to pump a Thulium-doped fiber. The input optical signals are monitored and the powers of the three pumps are adjusted based on the input optical signals to control the gain generated by the TDFA. This prior art TDFA was provided in a paper by Won Jae Lee et. al. entitled “Gain excursion & tilt compensation algorithm for TDFA using 1.4 μm/1.5 μm Dual Wavelength Pump Control” published in OFC 2002 (Lee Paper).
One problem with the TDFA described in the Lee Paper is that monitoring the input optical signals and adjusting the pumps based on the input optical signals may be a difficult and inaccurate method of controlling the gain of the TDFA. Another problem with the TDFA in the Lee Paper and other TDFA configurations is that the gain of the TDFA is not clamped. System designers may have a difficult time using the current TDFAs because the designers may not be able to anticipate the gain of the TDFA.